This invention relates to an improvement in the manufacture of commutators for dynamoelectric machines and in particular to the commutators for direct current motors and generators.
More specifically, this invention relates to the controlled removal of insulating material from between the conductor bars on a commutator through use of a high energy beam.
Commutators comprise an array of electrically and geometrically separated conductors, hereinafter also referred to as commutator bars, supported mechanically in a matrix of non-conducting material. The commutator bars are commonly made of copper; other metals have been suggested for special purposes. The non-conducting material may be any insulator, preferably one with high mechanical and dielectric strength and elevated melting points, such as bakelite, mica and certain epoxy resin compositions. The array of commutator bars may be arranged on the surface of a cylinder, a disc or a flat strip depending on the configuration of the electrical machine and the desired properties of commutation.
In a conventional D.C. permanent magnet motor, the commutator may take the form of a drum comprised of conductor bars aligned with the axis of the motor shaft and separated and supported by narrow, non-conducting spaces. Other commutators known as face commutators are comprised of commutator bar segments arranged in sequence on an insulating carrier disc with the interspace between the segments partially filled with the insulating material. For use with linear motors, the commutator may have the form of a flat strip comprising parallel, alternate conducting and non-conducting bars aligned at right angles to the direction of travel of the motor moving part.
For reliable and efficient commutation, the surface of each conductor bar is preferably smooth and regular so as to produce a minimum of brush friction and brush wear. For many applications it is desirable to avoid any contact between the brush material and the non-conducting material of the matrix supporting the conductor bars thus avoiding extensive abrasion as well as brush contamination. Furthermore, abraded material tends to accumulate between brush and commutator resulting in degradation of commutation with increased sparking, wear and power dissipation.
It is known to avoid contact between brush surface and the material of the non-conducting matrix by the so-called "undercutting process" which selectively removes some of the insulating material at the surface between conductor bars, so that the surface of the non-conducting material is recessed between and with respect to the surface of the conductor bars. Using a brush with a surface in contact with the commutator which is substantially greater than the width of the non-conducting interspace between bars, any undesired contact between brush and non-conductive material may thus be avoided.
In one typical process for the manufacture of commutators the copper commutator bars are potted in a resinous material, e.g., an epoxy resin having satisfactory dielectrical and structural strength. The potted assemblies are then machined, e.g., turned and polished to remove excess of resinous material from the commutator bars and to provide a smooth clean commutating surface. This leaves the surface of the resinous material flush with the surface of the conductor bars. In order for removing said insulating material to a degree sufficient for impressing its surface below the surfaces of the conductor bars, a slicing saw or an end-mill or similar mechanical instruments are employed as, e.g., described in U.S. Pat. No. 3,279,041. Sawing or milling away the excess insulating material requires close attention of a skilled machine operator and is a time-consuming, labor-intensive and difficult operation. In addition, the sawing and/or milling operation often produces burrs on the side edges of the conductor bars necessitating an additional step for their removal.